In a wavelength converter apparatus provided with its wavelength converting device for changing the wavelength of a fundamental wave emitted from a laser diode, the wavelength of the fundamental wave has to be held in a permissible range determined by the wavelength converting device in order to produce its light output of a desired level. However, a change in the driving current of the laser diode caused by controlling the light output converted at constant level, may shift the oscillation wavelength of the laser diode with a change in the resonator length or in the gain spectrum creating a mode hopping phenomenon. The mode hopping phenomenon causes a momentary change in the light output, hence disallows the control system to carry out its action of maintaining the light output to a uniform level, whereby the light output will exhibit variations or noises.
In order to compensate the instability, a technique for locking the wavelength of a fundamental wave is known. As one of the familiar techniques, fiber Bragg grating having a Bragg diffraction grating formed in the core of optical fiber is provided to act as a wavelength selective device in an extra-cavity for carrying out the action of optical feedback to the laser diode and selecting the oscillation wavelength (See Patent Document 1).
Also, another technique is known where a reflection type volume holographic grating is provided to act as the wavelength selective device in an extra-cavity for carrying out the action of optical feedback to the laser diode and selecting the oscillation wavelength (See Patent Document 2).
Patent Document 1: U.S. Pat. No. 5,485,481.
Patent Document 2: U.S. Pat. No. 5,691,989.
In case that the fiber Bragg grating is employed as the wavelength selective device in one of the foregoing conventional techniques, its thermal characteristic is found that its Bragg reflection wavelength affects on the action of converting the wavelength and its temperature has to be held at a uniform degree. It is however difficult to maintain the temperature of the optical fiber at a uniform degree, which is comparatively troublesome to be handled. In addition, when the optical fiber is loaded with a physical stress for controlling the temperature, its light output may significantly be disturbed in the polarized state. In general, as the polarization of incident light received by the wavelength converting device remains fixed, its disturbed form may directly affect the light output.
Also, in the other conventional technique where a reflection type volume holographic grating is employed as the wavelength selective device, the resonator mode of the laser diode and the extra-cavity mode of the wavelength selective device will compete with each other unless the length of the optical path extending from the laser diode to the reflection type volume holographic grating is set significantly longer than the coherence length. Accordingly, the oscillation wavelength of the laser diode can hardly be pulled to the center wavelength of the reflection type volume holographic grating. In case that the optical path extending from the laser diode to the reflection type volume holographic grating is spatially increased, its setting will enlarge the overall arrangement of the apparatus.
It is hence an object of the present invention to provide a wavelength converter apparatus arranged with its wave selecting device pulling wavelengths in stability while requiring no action of strictly controlling the temperature of the device in a relatively smaller structural arrangement thereof.